It is known to inspect semi-conductor components in an “ultraclean vacuum” (UCV) environment using extreme ultra-violet (EUV) light with a wavelength of 13.5 nm. A very low level of contaminants in the vacuum environment is necessary to obtain a desired operational life of a UCV/EUV inspection system. Specifically, components, such as optics and sensors, in the UCV/EUV inspection system must kept clean and the thickness of carbon or oxidation layers (haze) on the components must be held to no more than about 2 or 3 nm. For example, typical requirements for a UCV/EUV inspection system are contaminant levels in a range of 1-100 parts per trillion, high total pressure (0.1-10 Pa) with extreme low partial pressures of contaminants (˜10-12 mbar), and water partial pressure below 10−7 mbar.
However, haze formation on deep ultra-violet (DUV) and EUV optics and masks is a common problem. Various types of ammonium salts are typically identified as haze forming species; most common are ammonium sulfate, ammonium nitrate, ammonium phosphates, ammonium oxylate, and chlorine. In addition, organics and siloxanes are identified as haze formation species as well. It is known in the art that merely controlling and eliminating one type of the haze formation precursor and species results in the appearance of another type.
The performance of UCV/EUV inspection systems degrades at an unacceptable rate due to oxide formation and carbonaceous layer accumulation on surface of key components accelerated by DUV or EUV stimulated surface reaction in the presence of contaminants. That is, during operation of inspection systems, critical optical components (mirrors, windows, sensors/detectors) and surfaces including chamber walls must be clean to prevent deposition of contaminants. Thus, precise control of contaminant levels, for example of hydrocarbons, acids, bases, H2O, and O2, enhances cleanliness of a UCV/EUV inspection system and maximizes the life of components in the system. The sources of contaminates are from the inspection system itself and from contaminates in the system components.
It is known to use capping layers to minimize oxidation and contamination built up on components in a EUV inspection system. For example, the components are protected by a Ruthenium capping layer. However, under the typical operating conditions for a EUV inspection system, Ruthenium is still highly prone to oxidation and carbonaceous species built-up.
Conventional “ultra-high vacuum” (UHV) approaches, such as multi-stage pumping and high temperature baking, are used to reduce the respective partial pressures of O2, H2O and hydrocarbons. However, such methods are not feasible for UCV/EUV inspection systems. Specifically, key components in an UCV/EUV inspection system are damaged by the elevated temperature associated with high temperature baking
It is known to maintain cleanness of inspection systems by prequalifying material used in the system, rigorously cleaning and out gassing prequalified material, and maintaining the system under inert gas purged environment with specified O2, H2O content, and control of impurities, such as hydrocarbons, acids, bases, via filters. The preceding methods are not particularly effective and are cumbersome to implement. It also is known to directly evaporate alkali metal on a chamber surface. The thin film of the metal acts as a getter, slowly react with impurities. The alkali metal film is formed from liquid phase from alkali metal evaporation. However, the evaporation rate is difficult to control and liquid alkali metal is highly corrosive and mobile. The metal film formed has low purity since it is difficult to clean the target. It is known to use a mixture of alkali metal chromate with a reducing agent. However, this method results in a limited supply of alkali metal (the getter) and generates undesired by-products when releasing alkali metal.